The present invention relates to a system for locating terminals in a radio communication system and, more particularly, to the location of terminals in radio communication systems wherein spot beams are used to irradiate geographical regions to provide communication coverage.
One example of radio communication systems employing spot beams can be found in satellite communication systems. Another example is cellular/PCS systems which use fixed-beam phased arrays. To achieve an economically useful capacity to serve a large number of subscribers, communication systems need to allow reuse of the available spectrum many times over the globe. This can be achieved, for example, by the use of multiple spot beam antennas that divide the illumination of the chosen service area between many smaller regions.
The most promising satellite systems for such applications are those which place satellites in a low earth orbit (LEOs), in a medium earth orbit (MEOs) or in a stationary earth orbit (GEOs). Disadvantages of satellites in stationary orbits include the huge antennas that are needed to create the desired size spot beams from the 40,000 km orbit distance and the long delay in signals traversing the orbit distance which creates a problem for two-way conversations. On the other hand, the disadvantage of satellites in low earth orbits is that the satellites move relative to the earth and thus the areas that the spot beams illuminate change as the satellites circle the earth. Medium earth orbiting satellites exhibit the problems of both LEOs and GEOs, although to a lesser degree.
Satellite systems employing low or medium earth orbit satellites need to compensate for the rapidly changing propagation delay in the links between the satellites and terminals on the earth caused by the satellites' movement relative to the earth. During communication in such systems, Doppler compensation is provided to the signals to account for the change in propagation delay. In order to provide Doppler compensation, the terminal can search for the Doppler frequency which is an exhaustive and time consuming process that delays connection to the system. Alternatively, however, given knowledge of the terminal's location, as well as satellite ephemerides information, a Doppler compensation can be calculated instead of searching for the Doppler frequency, thereby speeding up the process of call origination.
Knowledge of the terminal unit's position can also be useful in the provision of many other system functions. For example, handover of a terminal unit between spot beams and channel assignment can be facilitated using knowledge of the terminal's position. Another example is public safety systems and 911 services wherein the ability to locate terminals in emergencies is important. Moreover, as described in the above-identified U.S. Pat. No. 5,619,503, terminal position is also useful in computing the beamforming matrix.
A conventional method for determining a terminal's location is to use information derived from the Global Positioning System (GPS). The GPS includes a number of MEO satellites, each of which transmits a known signal. From any given point on earth, a terminal can receive and measure three or four such signals (because of the large number of GPS satellites in orbit) to determine a time delay and hence a distance between the three or four satellites and the terminal. This information can then be used to triangulate on the terminal's position. Although this technique is reasonably effective in a system in which many different satellites' signals are available to a receiver, other systems, such as GEO satellite systems, do not have this luxury. Accordingly, it would be desirable to provide different methods and systems for obtaining position information for terminals in radiocommunication systems which do not rely on the availability of signals from multiple satellites. One set of such systems and methods are provided in the above-identified pending U.S. patent application Ser. No. 08/597,073 which describes techniques whereby signals generated at a satellite using multiple phased array antenna beams can be used to generate position estimates for a mobile station. However, it would also be desirable to locate mobile stations in systems which provide radio communication using land-based stations as alternatives to (or in conjunction with) satellites.